In order to guide a robot arm in a welding robot it is necessary to locate the seam to be welded relative to the welding electrode. It is desirable to obtain three dimensional (3D) information concerning the topology of the target surface in which the seam lies since this enables much more information to be obtained about the surface and attributes much greater versatility to the welding robot.
An imaging device capable of obtaining 3-D information about a target surface has been proposed by the Caterpillar Tractor Co., in the U.S. Pat. No. 4,501,950. In this document, the disclosed imaging device comprises a support structure supporting a laser beam source, arranged to direct an incident laser beam into a beam guide whereby the laser beam is directed to traverse a circular path on the target surface. The beam guide is also provided with a window which rotates synchronously with the incident laser beam in order to view an image of the target surface illuminated by the laser beam. The window forms part of a receiving portion of the beam guide which includes an assembly of mirrors or prisms arranged to reflect the image into a chamber which extends along the axis of rotation of the beam guide, and to direct the image through the chamber. A dove prism is mounted in the chamber and supported by a sub-assembly of the beam guide to rotate at half the speed, but in the same direction, as the projecting portion of the beam guide. Thus, as the image passes through the dove prism it is " de-rotated" with respect to the support structure. Upon leaving the dove prism the image is directed into a camera assembly fixedly mounted above an aperture in the chamber.
The camera assembly of this device includes a focusing lens arrangement which directs the "de-rotated" image onto a one-dimensional sensor array. The sensor array is sensitive to the linear position at which the laser beam strikes it, and transduces this information into electrical signals which are transmitted to a data processor. The information is processed in conjunction with the angular orientation of the light guide which corresponds to the position of the laser beam on the circular path. The information concerning the angular orientation is obtained from a substantially conventional transducer.
It will be appreciated that it is highly desirable that an imaging device located on the end of a robot arm should be as light and compact as possible. However, it is also necessary to accommodate the conflicting requirement of being robust, so that the imaging device can endure the very hostile conditions experienced during a welding operation. The quality of the optics, particularly the light guide, dramatically affects the performance of the imaging device as a result of losses and distortion in transmission.
Some of these problems can be alleviated by omitting the optical "de-rotation" of the image, and using a two dimensional light sensor in the camera assembly instead and devising a suitable program for "de-rotating" the image by computer, as suggested in the aforementioned U.S. patent. However, the difficulties of processing the information obtained, and the loss of immunity to multiple reflection problems in particular, is often such as to outweigh the advantages of this tactic.
The complexity of imaging devices constructed in accordance with U.S. Pat. No. 4,501,950, results in them being large, heavy, difficult to set up, expensive and lacking in robustness. Hence the versatility of the prior art imaging devices is severely limited.
It is an objective of the present invention to provide an imaging device suitable for mounting on the arm of a welding robot, close to the welding torch, in order to obtain 3-D topological data from a target surface, which alleviates the aforementioned disadvantages of the prior art.